1. Field of the Invention
Embodiments of the invention generally relate to a refillable reference electrode used in chemical analysis.
2. Description of the Related Art
Monitoring and/or determining the composition of a plating solution during an electrochemical plating (ECP) process is problematic, as the depletion of certain additives is not generally constant over time, nor is it generally possible to correlate the plating solution composition with the duration of the plating solution use. As such, it is difficult to determine the amount of additives in a plating solution with any degree of accuracy over time, as the level of additives may either decrease or increase during plating, and therefore, the additive concentrations may eventually exceed or fall below a tolerance range for optimal and controllable plating. Conventional ECP systems generally utilize a cyclic voltammetric stripping (CVS) process to determine organic additive concentrations in an ECP solution.
In a CVS process, the potential of a working electrode is generally swept through a voltammetric cycle to estimate an unknown additive concentration. More particularly, three electrodes, a working electrode, a counter electrode, and a reference electrode, are immersed in a cell having plating solution to be measured therein. The reference electrode and the working electrode are typically connected to a device for measuring the electrical potential difference between the respective electrodes. The reference electrode generally consists of three components, a half-cell electrode, a half-cell electrolyte, and a reference junction. As used herein, the term “half-cell electrode” generally refers to a solid phase, electron conducting contact within the half cell electrolyte, at which contact a half-cell oxidation-reduction reaction occurs that establishes a stable potential between the half-cell electrolyte and the working electrode. Direct physical, and therefore electrical, contact between the half-cell electrolyte and the sample plating solution is established through the reference junction, which usually consists of a porous ceramic or metal plug, or other device capable of achieving a fluid mechanical leak. The reference junction is generally necessary to establish electrical contact with the plating solution, and therefore, the working electrode. To prevent contamination of the electrolyte by the plating solution through the reference junction, electrolyte generally leaks from the reference junction into the plating solution. Therefore, the electrolyte solution generally has to be refilled intermittently with liquid electrolyte to maintain a set level of electrolyte in the reference electrode. Conventional reference electrodes generally have an axial hole with a circular cross-section for use as a refill channel, which may be sealed with a plug during analysis.
However, many challenges are associated with refilling conventional reference electrodes. For example, if the electrolyte level falls below a minimum level, the reference electrode will provide unstable results, and therefore, the reference electrode is generally refilled manually, which provides additional problems. Operator errors, such as not promptly refilling the reference electrode, lead to unreliable results. In addition, removing and reinstalling the reference electrode may wear out the electrical contact and may also lead to a noisy response from over tightening the electrode. An approach to eliminate the problem of refilling electrolyte solutions in reference electrodes has been to employ gel filled reference electrodes, which generally do not require refilling. However, gel filled solutions may become contaminated by ion migration into the gel and by depletion from ion migration out of the gel, which results in sensor drift. As such, there is a need for a method and apparatus for refilling reference electrodes, wherein the method and apparatus are not susceptible to the consequences of conventional reference electrode refilling procedures.